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 Scientists are hoping Dawn will shed light on the early solar system by exploring an asteroid and a dwarf planet.
It is the world's first attempt to journey to a celestial body and orbit it, then travel to another and circle it as well. Ion-propulsion engines, once confined to science fiction, are making Dawn's trip possible.
"To me, this feels like the first real interplanetary spaceship," said Marc Rayman, chief engineer. "This is the first time we've really had the capability to go someplace, stop, take a detailed look, spend our time there and then leave."
The 3 billion-mile trip began with a liftoff a little after sunrise.
Dawn won't reach Vesta, its first stop, until 2011, and Ceres, its second and last stop, until 2015.
Vesta and Ceres are the two largest bodies in the asteroid belt between Mars and Jupiter. In fact, just last year, Texas-size Ceres was upgraded to a dwarf planet. Vesta, still an asteroid, is about the length of Arizona.
Scientists chose the two as targets not only because of their size, but because they are so different from one another.
Not quite spherical Vesta is dry and rocky, and appears to have a surface of frozen lava. It's where many of the meteorites found on Earth came from. Nearly spherical Ceres is icy and may even have frost-covered poles. Yet both were formed around the same time in the planet-forming stage of the solar system 4 1/2 billion years ago.
Spacecraft have flown by asteroids before -- albeit much smaller -- and even orbited and landed on them, and more asteroid missions are on the horizon. But none has attempted to orbit two on the same mission, until Dawn.
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 "While these other asteroid missions are, I think, very exciting, I hope one doesn't confuse the kind of asteroids that Dawn is going to with the near-Earth asteroids and these other small bodies," said Rayman, who is based at Jet Propulsion Laboratory in Pasadena, Calif. "I think many people think of asteroids as kind of little chips of rock. But the places that Dawn is going to really are more like worlds."
Dawn has cameras, an infrared spectrometer and a gamma ray and neutron detector to probe the surfaces of Vesta and Ceres from orbit. It also has solar wings that measure nearly 65 feet from tip to tip, to generate power as it ventures farther from the sun.
Most importantly, Dawn has three ion engines that will provide a gentle yet increasingly accelerating thrust. Electrons will bombard Dawn's modest supply of xenon gas, and the resulting ions will shoot out into space, nudging the spacecraft along.
Even "Star Wars" had only twin ion engines with its T.I.E. Fighters, Rayman noted with a smile earlier in the week.
The mission costs $357 million, excluding the unpublicized price of the Delta II rocket that hoisted Dawn.
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On the Net:
Jet Propulsion Laboratory: http://dawn.jpl.nasa.gov/
[Associated Press; by Marcia Dunn]
Copyright 2007 The Associated Press. All rights reserved. This
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